Statutory requirements provide for an onboard diagnosis of motor vehicle components such as catalytic converters which are relevant for toxic substance emissions. In this context, U.S. Pat. No. 4,622,809 discloses applying the signal amplitude of an exhaust-gas probe to evaluate the state of the catalytic converter. The exhaust-gas probe is sensitive to oxygen and is mounted rearward of the catalytic converter. The known method is based on the averaging effect with which the catalytic converter operates on the oxygen content in the exhaust gas. If the exhaust gas flowing into the catalytic converter is rich in oxygen, then the catalytic converter can store excess oxygen within certain limits and release the oxygen when the exhaust-gas entry is deficient in oxygen. An oscillation of the oxygen content in the exhaust gas forward of the catalytic converter is therefore attenuated by the catalytic converter, that is, the amplitude of the oscillation is reduced. The oscillation of the oxygen content in the exhaust gas typically takes place in a known strategy for the control of the air/fuel ratio for an internal combustion engine.
If the exhaust-gas probe, which is mounted rearward of the catalytic converter, registers an unexpectedly intense oscillation of the oxygen content, this is evaluated as an indication of a defective catalytic converter because a loss of the toxic substance conversion capability accompanies the loss of the attenuating effect.